Inches to MM Converter
Convert inches to millimeters in one step. Enter a length in inches and get the exact value in mm, with the formula, examples and a fractional-inch table below.
Millimeters = Inches × 25.4
Inches to MM Formula
To convert inches to millimeters, multiply the number of inches by 25.4. The factor is exact, so there is no rounding error in the conversion itself. To convert millimeters to inches, divide by 25.4. Because the factor is a simple decimal shift of 2.54 cm, you can also multiply by 2.54 and then move the decimal point one place to the right.
Inches and Millimeters Explained
The inch (in) is the imperial and US customary unit used for screen sizes, pipe and fastener sizes, tyre widths and many tools. One inch is defined as exactly 25.4 millimeters. Inch measurements are often written as fractions such as 1/4, 3/8 or 5/16, which makes decimal conversion a frequent first step.
The millimeter (mm) is one thousandth of a meter. It is the standard unit for precise work in engineering, manufacturing, 3D printing and technical drawings, where a tenth of a millimeter can matter.
How to Convert Inches to Millimeters
- Enter the length in inches. Decimals such as 0.375 work, and negative values are accepted for tolerances and offsets.
- The answer in millimeters appears instantly, or press Convert Now.
- Use Swap Units to convert millimeters to inches, Copy Result to copy the answer, or Clear to start again.
Step-by-step by hand
- If the size is a fraction, divide it first: 5/16 in = 5 ÷ 16 = 0.3125 in.
- Multiply by 25.4: 0.3125 × 25.4 = 7.9375.
- Add the unit: 5/16 in = 7.9375 mm.
For a quick estimate, multiply by 25 and add 1.6%. For machining and drawings, keep all the digits until the final step and round only the finished dimension.
Practical Examples
- Rainfall: 1 inch of rain is 25.4 mm, and a 3.5 in storm total is 88.9 mm.
- Bolt and drill sizes: a 1/4 in bit is 6.35 mm, and a 3/8 in bit is 9.525 mm.
- Gaps and spacing: a 1/8 in gap is 3.175 mm, and 3/4 in spacing is 19.05 mm.
- Tolerance: ±0.005 in is ±0.127 mm.
- Device thickness: a tablet 0.3 in thick measures 7.62 mm.
- Negative value: −2 in = -50.8 mm.
Inches to MM – Common Values
Quick reference for frequently used conversions
| Inches (in) | Millimeters (mm) | Centimeters (cm) |
|---|---|---|
| 1/32 in | 0.79375 mm | 0.079375 cm |
| 1/16 in | 1.5875 mm | 0.15875 cm |
| 1/8 in | 3.175 mm | 0.3175 cm |
| 1/4 in | 6.35 mm | 0.635 cm |
| 3/8 in | 9.525 mm | 0.9525 cm |
| 1/2 in | 12.7 mm | 1.27 cm |
| 5/8 in | 15.875 mm | 1.5875 cm |
| 3/4 in | 19.05 mm | 1.905 cm |
| 7/8 in | 22.225 mm | 2.2225 cm |
| 1 in | 25.4 mm | 2.54 cm |
| 1.5 in | 38.1 mm | 3.81 cm |
| 2 in | 50.8 mm | 5.08 cm |
| 3 in | 76.2 mm | 7.62 cm |
| 4 in | 101.6 mm | 10.16 cm |
| 6 in | 152.4 mm | 15.24 cm |
| 8 in | 203.2 mm | 20.32 cm |
| 12 in | 304.8 mm | 30.48 cm |
| 24 in | 609.6 mm | 60.96 cm |
| 36 in | 914.4 mm | 91.44 cm |
Where and Why This Conversion Is Used
Millimeters are the language of precision, so any time an inch-based size meets a metric specification, this conversion is needed.
- Machining and engineering: imported parts, drawings and standards may list dimensions in inches while your tools and tolerances are metric.
- 3D printing and CAD: models downloaded with inch units must be scaled to millimeters before slicing.
- Hardware and plumbing: bolts, drill bits, pipes and fittings are sold in inch sizes in North America and mm sizes elsewhere.
- Weather: rainfall and snowfall totals are reported in inches in the US and millimeters in most other countries.
- Electronics and photography: sensor, lens and display specifications often mix inches and millimeters.
For the reverse direction use the MM to Inches Converter, or convert to centimeters with the Inches to CM Converter.